1. Field of the invention
This invention relates to a disc molding apparatus, and more particularly an apparatus for molding a disc-like information medium such as an optical disc, a photomagnetic disc or the like.
2. Description of the Prior Art
Conventionally, a disc-like information medium such as an optical disc, a photo-magnetic disc or the like (hereinafter referred to as "disc") which functions as an information recording carrier is generally formed by injection molding using a molding apparatus for a disc. In such a conventional disc molding apparatus used for this purpose, ejection of a molded disc product for release of the disc from a stamper is conducted in such a manner as shown in FIG. 1. More particularly, it is carried out by blowing air against a molded disc 100 through an air blowout slit 102 defined between a stamper holder 104 arranged at a movable mold 106 and a sleeve 108.
In general, a shape of grooves or pits formed on a transfer surface of a disc by transfer-molding is classified into two groups. One of them is a spiral group wherein a continuous groove of uniform unevenness is spirally formed from an outer periphery of the disc to its inner periphery. The other is a group wherein pits are partially discontinuously arranged as used in a sample servo system in which only some pits different in length are formed. This is typically seen in a compact disc or the like.
Formation of a substrate for a disc by injection molding using a stamper having a format capable of providing the disc with such a transfer surface as described above causes traces like slippage of pits or grooves to occur adjacent to positions at which the pits or grooves are to be originally formed. More specifically, in the conventional disc molding apparatus, the air blowout slit 102 is positioned at a central region of the molded disc, so that it is required to carry out peeling of the disc from a stamper 110 at the central portion of the disc 100. This causes failure in the proper release of the disc from the stamper and adversely affects the transfer surface of the disc during the release., resulting in birefringence non-uniformity, deformation of grooves or pits formed on the transfer surface of the disc, deformation of the disc itself, distortion of the disc and the like to deteriorate quality, reliability and productivity of the disc.
Also, when a molded disc is formed of a polycarbonate resin material, it is required to take out the molded disc product at a temperature as high as about 120.degree. C., resulting in further deformation of the molded disc. Further, an area ratio between projections and depressions between grooves or pits formed on a transfer surface of the disc is significantly large, so that a degree of adhesion between the resin and the stamper at the projections is highly different from that at the depressions to lead to deformation of the substrate, resulting in the molded substrate separated once from the stamper being contacted with the stamper again during the release as shown in FIG. 2. Thus, deformation of the molded disc is further promoted to deteriorate quality, accuracy and reliability of the molded disc to a degree sufficient to cause a laser to fail to follow tracks on the molded disc.
Accordingly, it would be highly desirable to develop a disc molding apparatus which is capable of forming a disc with high accuracy and without any deformation and improving productivity in the molding of the disc.